Actually there are 6 reasons why avalanches are caused.
1. Layers of snow build up and slide down the mountain.
2. A vibration or movement like the voice of a person or a train can star an avalanche.
3. Certain weather conditions like wind and rain.
4. A rock or a piece of ice can shake the snow to slide down the mountain.
5. Water under the snow could make the snow slide.
6. A section of the snow could fall down or break apart and slide down the mountain.
aReader (see comments for their answer) came the closest. They got #2. Very good to you.*
The other question was are snowflakes ever the same shape. No one ever answered that one but it is quite simple.
The answer is no.
*To learn about names in question of the month see the page about ‘Things You Might Need To Know About My Blog” or click here.